1. Field of the Invention
The present invention relates to the fields of chemistry, photolithography and semiconductor fabrication. More specifically, the invention is directed to a composition of a high contact angle topcoat material and a method of forming a photolithographic image using the topcoat, where a liquid such as water is interposed between the last lens fixture of an exposure tool and the photoresist-coated wafer.
2. Description of Related Art
The continuous drive to print smaller structures for advanced electronic device manufacturing requires the use of higher resolution optical lithography tools. Immersion lithography has the potential to extend current 193 nm argon fluoride-based technology to 45 nm critical dimensions (half-pitch DRAM) and beyond by effectively improving the depth-of-focus processing window for a given optical numerical aperture (NA). In addition, it enables lens designs with NA greater than 1.0, thus resulting in an increased resolution of optical scanners. The process requires filling the gap between the last lens element of the exposure tool and the resist-coated substrate with ultrapure water or another suitable fluid. See A. Hand, “Tricks With Water and Light: 193 nm Extension”, Semiconductor International, Vol. 27, Issue 2, February 2004.
One of the technical challenges facing liquid immersion lithography is the diffusion between the photoresist components and the immersion medium. That is, during the immersion lithographic process, the photoresist components leach into the immersion medium and the immersion medium permeates into the photoresist film. Such diffusion is detrimental to photoresist performance and might result in disastrous lens damage or contamination in a 40 million dollar lithography tool. Therefore, there is a need for a method to prevent interaction between photoresist layers and immersion fluid in an immersion lithography system.
Topcoat materials can be applied on top of the photoresist layer for the purpose of eliminating diffusion of materials from the photoresist layer underneath, and to prevent the permeation of the exposure medium into the photoresist film. Traditionally, topcoat materials have been used in photolithography as anti-reflective films on top of a photoresist. The top anti-reflective coat (TARC) materials can prevent the multiple interference of light that takes place within the photoresist layer during exposure. As a result, the critical dimension (CD) variation of the geometrical features of a photoresist pattern caused by the variation in the thickness of the photoresist film can be reduced.
For ease of processing, classic TARC materials are designed to be soluble in both water and aqueous base developers so that they can be applied directly from water solution and subsequently removed by the aqueous base developer during the develop stage.
Numerous TARC materials have been developed previously. For example, U.S. Pat. Nos. 5,744,537 and 6,057,080 disclose aqueous-soluble TARC materials comprising a polymeric binder and a fluorocarbyl compound, which have nearly ideal refractive indices on the order of 1.3-1.4. U.S. Pat. No. 5,879,853 also discloses a TARC material that is removable by a wet process. U.S. Pat. No. 5,595,861 similarly discloses a TARC comprising partially fluorinated compounds, which can also be water soluble. U.S. Pat. No. 6,274,295 discloses a TARC material comprising a light-absorbing compound having a wavelength of maximum absorption higher than an exposure wavelength used to expose the photoresist. This TARC can also be water-soluble. Finally, U.S. Pat. No. 5,240,812 discloses a protective material for use as an overcoat film for acid catalyzed resist compositions to prevent contamination from vapors of organic and inorganic bases. While not specifically disclosed as being a TARC, the overcoat can also be water-soluble.
Since water has been proposed as the exposure medium for 193 nm immersion lithography, classic water-soluble TARC materials such as those described above cannot be used as topcoats for such technology. Other commercial materials currently available either require solvents that are incompatible with semiconductor fabrication lines or impact the lithographic performance of the photoresist. New topcoat materials are needed to ensure the deployment of 193 nm immersion lithography necessary for manufacture of semiconductor devices at 45 nm and below design ground rules. See M. Slezak, “Exploring the needs and tradeoffs for immersion resist topcoating”, Solid State Technology, Vol. 47, Issue 7, July 2004.
At IBM, organic polymer-based topcoats or hybrid (organic-inorganic) molecular topcoats are being developed. This is a new area of exploration and there is a need to develop a variety of topcoats that are compatible with different resists, or ultimately functions as universal topcoats. A critical deficiency of most of the reported base soluble topcoats is that they have relatively high surface energy, resulting in a “low” water contact angle (in the range of 70-80 degrees). It has been reported low surface energy (contact angle greater than 90° C.) topcoats may be important for high-speed scanning and low defectivity in immersion lithography. If immersion lithography realizes its potential as a successor to dry 193 nm lithography in volume production, the combination of high speed scanning and low defectivity are essential.
In fact, the first working topcoats were based on Teflon-AF-like Fluoropolymers and have very low surface energy and have water contact angles in the 110-120 degree range. However, these topcoats were not developable in aqueous base and led to involved/expensive/impractical processing.
Thus, there remains a need for a topcoat material that is highly soluble in developer, resistant to immersion fluid, compatible with photoresist, and has low surface energy with water contact angle greater than 90° C.